The present invention relates to a method for the thermal treatment of metallic workpieces, in particular workpieces with an overset or undulating shape or workpieces that are projectingly stackable, in which method the workpieces, after being heated, are cooled in a quenching chamber with a quenching gas. The present invention furthermore relates to an apparatus with which such a method can be performed.
Metallic workpieces undergo thermal treatment in order to produce defined workpiece properties, for example, a high degree of hardness or sufficient resistance to wear. The result of the thermal treatment is a change in the structure of the workpiece, for example, a conversion of the cubic, surface-centered γ-structure of carbon-rich austenite plates into the cubic space-centered α-structure of ferrite plates. In addition to temperature and original structure, particularly important in terms of the results of the treatment are the speed at which the heated workpieces are cooled and the quenching means used. Quenching means primarily used are gas, oil, and Water (given in order of increasing abruptness).
When gas quenching is used, relatively high speed is needed in order to achieve a quenching intensity that approaches the quenching intensity of oil or water quenching. For this purpose, it is known to provide nozzles with which correspondingly high gas speeds and therefore generally sufficient heat transmission values of greater than 1000 W/m2K can be achieved. An apparatus provided with nozzles for quenching metallic workpieces is disclosed, for example, in EP 0 796 920 A1. This apparatus, having a field of nozzles made of an exchangeable nozzle plate provided with nozzles, uses a rotating or swiveling arrangement of nozzle plate and/or of a grate carrying the workpieces. This makes possible relative movement of nozzle field and workpieces and thus ensures that the gas is applied to the workpieces in a relatively uniform manner, but it has the disadvantage that the flow of gas strikes the workpieces in a diffuse and turbulent manner. This means that the surface of each workpiece is cooled unevenly, so that stresses occur that can lead to distortion or even fissures. In particular with workpieces with an offset or undulating shape, or those that are projectingly stacked when a lot or charge is assembled, for example, roller bearing rings or toothed wheels, distortion is particularly noticeable and, due to the generally narrow tolerances for such workpieces, leads not infrequently to a number of rejects that is unsatisfactory in terms of economics.
It is well-known for systematically controlling the flow of gas, to provide guides with which, for example, the quenching gas can strike even those locations of the workpiece surface that would otherwise be located in the shadows of the flow; however, such a measure is not satisfactory because, in addition to the relatively high degree of complexity associated with arranging the guides to correspond to the workpiece geometry, the guides do not prevent eddies from occurring when the flow of gas strikes the workpieces, which can have a negative effect on how adjacent workpieces in a charge cool and can thus cause distortion.
It is therefore an object of the present invention to provide a method and an apparatus for thermal treatment of metallic workpieces with which gas quenching can be obtained that is low in distortion, even for workpieces having an undulating shape or workpieces that are projectingly stackable.